Sigma 100-300
 

Just bought a Sigma 100-300 as an upgrade from the Sigma70-300. The lens works fine on the D70 but refuses to Auto focus when I use the Kenko Pro300 1.4x teleconverter. Using the Kenko with the 70-300 is no problem.

So have I bought a lemon or is there a mod that I can do to get the 100-300 working with the kenco.

Any help with this issue would be much appreciated.
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Which 100-300 lens is it? Have you gone for the one with the constant f4?

WPF Admin Andy's site has an article on using teleconvertors here:
http://www.digiscoped.com/teleconverters.html
Note the last sentence of the 2nd paragraph. Even if you have an effective aperture of f/5.6 or bigger, you will need some edges/contrast for the AF to work, especially if the light levels are low.

Duncan.

Just to chime in a bit here ... a few things haven't been said clearly.

Many camera auto-focus systems refuse to work when presented with a lens of less than f/5.6. The top-of-the-line Canon "1" series cameras will happily auto-focus down to f/8. Canon 300D, 350D, 10D and 20D all want f/5.6 or better.

I shoot with Canon, I don't know what happens with Nikon (or other) equipment.

With the Canons there is a way to 'trick' the camera regarding the maximum aperture of the lens. A bit of tape over 3 of the electrical contacts between the lens and the converter will prevent the converter from being reported. This leaves the camera believing that it is using the straight lens and it will at least try to auto-focus. I say try because, due to the dimmer light with the teleconverter, it may not be successful.

A second thing that kills auto-focus (regardless of teleconverter use) is the lack of light or low contrast targets. Of course some cameras handle this situation better than others.